The relationship of resultant dipole moment (RDM) to heart size will be investigated using experimental animals. A quantitative determination of RDM magnitude, orientation and location will provide a measure of cardiac size superior to that obtained from scalar leads or simple vectorcardiographic systems. The method accounts for body size, shape, and conductivity and is relatively independent of electrode positioning errors and heart position. Effects of intracardiac blood, cardiac activation pattern, and respiratory variation will be considered in the analysis of results. The resulting RDM will be a measure of ventricular mass. A lead system for measurement of RDM is operational as are computer systems for processing the results. In this investigation, changes in measured RDM magnitude, orientation and location will be tested using artificial dipoles on ventricular walls of young pigs. Effects of increasing one dipole amplitude relative to the other will be determined as will the effect of intracardiac blood on RDM. Ventricular hypertrophy will be produced in rabbits by pulmonary and aortic stenosis, and a combination of hypertrophy and dilatation by aorta-caval fistula. Effects on RDM will be determined and compared with artificial dipole results to see if hypertrophy effectively increases the generator amplitude of the enlarged wall. Finally, the neonate pig will be studied as a model having a right to left ventricular mass ratio that decreases rapidly with age. Since cell proliferation persists early in post-natal life, the effects of cell number and cell size on RDM can be studied. As the measurement of RDM is non-invasive, the method can be used clinically both in adults and in infants, subsequent to the determination in this investigation cf the relationships between RDM ventricular size and ventricular mass.